[b]1 Characteristics of the Test Device from the Perspective of Electronic Circuit Structure[/b] 1.1 Human-Machine Interface Domestically produced test devices have Chinese menus for accessing various specialized test programs. The program interface is user-friendly and includes a help menu, making it very convenient and easy for general relay protection personnel to master. Imported test devices have English menus, making them equally convenient for personnel with a certain level of English proficiency. 1.2 Parameter Setting Parameters are directly input or modified through the screen. Generally, test devices must be pre-adjusted at the factory so that the output current and voltage signals are the set values. However, since the test device generally lacks instrument monitoring during automatic testing (such as the bisection asymptotic method), the set values ​​are assumed to be the output values ​​regardless of the device's operating conditions in subsequent tests, which presents a significant problem. The following are several methods currently used to solve this problem. (1) Foreign test equipment, such as FREJA RTS21, uses special accessories to calibrate the test equipment before each test and can make certain corrections with software. For example, OMICRON CMC156 uses A/D measurement automatic feedback correction. Its advantage is that it can improve accuracy when the power amplifier stage is not yet saturated. If the load impedance is large, the error of automatic feedback correction will increase after the power amplifier stage is saturated. However, the test equipment can issue an alarm. For example, when CMC156 checks the A phase current source, the error is close to zero when the output I = 0.05~9 A/load 1 Ω, but when I = 10 A/load 1 Ω, the waveform is clipped and the error reaches 2%, and a warning is issued. (2) Jiangxi Huadong Electric Power Instrument Factory JJC@ adopts a more proprietary method, which is equivalent to the two meters of the hardware circuit determining the pass, so that when the error between the instantaneous value (excluding zero point) of the output current and voltage and the set value exceeds 3%, an audible warning is issued, and when it exceeds 5%, the machine is shut down and the error phase is indicated to avoid incorrect setting. The JJC@ test device not only has open circuit and short circuit protection for the current and voltage circuit, but this distortion discrimination measure also strengthens the open circuit and short circuit protection of the current and voltage power supply. Moreover, when the output signal has an error relative to the computer setting signal, a warning can be issued. (3) Other domestic test devices have not taken preventive measures for distortion discrimination, such as the Angli test device. 1.3 Program control signal source Most test devices currently transmit the instantaneous value data calculated in the microcomputer to the D/A through the data communication line to generate a signal source. Some transmit step by step, and some transmit in batches. This is closely related to the number of test waveform points per week. Some test devices have only 40 points per week, while others have 180 to 240 points per week. Some have as many as 3,600 points per week. The signal source has a large number of data points per week, and the description is detailed. It does not need to use resistor-capacitor components for smoothing, so the amplitude-frequency characteristics are naturally good. The signal source has fewer data points per week, and the description is rough. It needs to use resistor-capacitor components for smoothing, resulting in very poor amplitude-frequency characteristics. Therefore, the output frequency converter voltage amplitude cannot be quantified according to the computer setting value, and it cannot be quantitatively tested for superimposed harmonics. (1) Beijing Weite MRT-02B and Guangdong Angli 3100D only have 40 points per week. They use transformerless output, which comes at a great cost. Their purpose is to improve the amplitude-frequency characteristics. However, the number of points per week is too small, and they are forced to use resistor-capacitor smoothing, which greatly reduces the amplitude-frequency characteristics. This is not advisable. (2) JJC@ test device has transformerless output and 180 points per week. The amplitude-frequency characteristics are greatly improved. When superimposing the 2nd, 3rd, 5th and 7th harmonics, the error between the output value of the harmonics and the computer setting value does not exceed 3%. When superimposing the 11th harmonic, the error is 10%. (3) Foreign testing devices such as OMICRON CMC156 or FREJA RT21 have a large number of points per week, estimated to be around 3,600 points. The output waveform is very smooth and the amplitude-frequency characteristics are very good. At the 20th harmonic, the error between the output value of the harmonic and the computer setting value is less than 3%. 1.4 Data conversion D/A The digital-to-analog converter generally uses 12 bits or more. In impedance element testing, the voltage signal may drop very low. At this time, the effective number of bits of the D/A converter of the signal source will be reduced, which will affect the number of steps of the waveform. Since the phase voltage is generally only 58 V, some testing devices are designed with the values ​​of the three voltages as 150 V, but this is unnecessary and wastes the resolution of the D/A converter. If the amplitude margin of the superimposed harmonic is considered, it is generally appropriate to take 80 V for the phase voltage. The zero-sequence 3Uo voltage should generally be designed as 100-150 V. 1.5 There are currently two designs for power amplifier stages. One is a transformer-based output. Its advantage is that impedance matching is achieved through the transformer, fully utilizing the capacity of the power amplifier tubes. Its disadvantage is that it cannot transmit non-periodic components, and its transient response and amplitude-frequency characteristics are poor. The other design is a transformerless output, which has a better transient response. Especially with the development of electronic device technology, the high-voltage drop and the price reduction of high-current power amplifier tubes have made the use of transformerless output the current trend. There are many key technologies in power amplifier stage design, which are also related to the hardware investment of the manufacturer. For experimental equipment, the current stage power amplifier has the highest capacity requirement. The different parameters of the power amplifier tubes selected in the power amplifier stage will result in different capacity ranges provided by the manufacturer. (1) The parameters of the power amplifier tube selected by JJC@ are: when the maximum output power is 300 VA/phase and the current is 30 A/phase, the voltage output to the load without distortion is 10 V. When the ambient temperature is 20 ℃, the current of 30 A is allowed to last for 5 min. When the current is 10 A/phase, the voltage output to the load without distortion is 15 V. At this time, it is allowed to work for a long time. That is, the power amplifier stage is limited by the amplitude of the DC power supply voltage. No matter what the current is, the waveform output must not be distorted. Otherwise, the waveform distortion judgment device will issue a warning or trip. (2) The parameters of the OMICRON CMA56 power amplifier device, as tested and verified by Jiangsu Provincial Electric Power Research Institute, are as follows: maximum output power: 150 VA/phase; when the current is 0-15 A/phase, the undistorted voltage output to the load is 10 V; when the current is 20 A/phase, the allowable voltage output to the load is 6.25 V; when the current is 30 A/phase, the allowable voltage output to the load is only 3.85 V; when the current is 50 A/phase, the allowable voltage output to the load is only 1.413 V; when the ambient temperature is 20 ℃, the allowable voltage output at 50 A/1.4 V is 8 min. (3) The parameters of Beijing Weite MRT-02B are: current 0-30 A/phase; long-term allowable operating current is below 10 A/phase; when the output current is greater than 10 A/phase, the test time should not exceed 10 s. 1.6 Automatic Loading Test According to the requirements of relay protection test items, the test device can output current and voltage according to the set program. For example, in the differential relay magnetizing characteristic test, it is necessary to output both power frequency current and DC current simultaneously, and automatically find the operating boundary asymptotically using the binary method. If the DC magnetizing current is selected as 20 A, the AC operating current will be close to 30 A. Domestic test devices generally operate in this way. Therefore, the allowable duration under high current should not be too short. Foreign test devices, such as FREJA RTS21, initially used a grid-based "carpet bombing" method when testing distance protection operating zones, using "." and "+" to indicate whether it operated or not. This method could not form the critical operating boundary, and it has now been improved to use the binary method asymptotically. 1.7 Relay Protection Feedback Information Relay protection feedback information can be of various types, such as dry contact, potential type, pulse type, and also have positive and negative polarity distinctions. Early test devices had poor intelligent discrimination, requiring hardware switch position changes to determine the processing of feedback information, which was inconvenient. Currently, some have achieved automatic intelligent discrimination, which is very convenient. This characteristic should also be considered when selecting test devices. 1.8 Test Data Processing: For distance protection operating zone tests, the automatic bisection method is used to asymptotically find the operating boundary, obtaining a series of test results. These test points are then plotted on the screen. Currently, many test devices cannot use intelligent fitting methods to plot the operating zone curve, nor can they obtain the measured operating impedance, thus failing to express the error between the measured impedance and the setting value. We have equipped the JJC@ microcomputer relay protection tester from Jiangxi Huadong Electric Power Instrument Factory with the following software: intelligent curve fitting programs for impedance element operating zone curves, precision current curves, and differential protection magnetizing characteristic curves, etc. Especially when testing the influence of superimposed harmonics on the impedance element operating zone, this program can effectively depict the non-circular operating zone curve. Foreign test devices use theoretical operating zones and simultaneously plot an allowable zone of ±5% error. If all test points fall within the allowable zone, it indicates that the test results are within tolerance, which also solves some problems. Domestic test device test reports generally save the data and print reports in Chinese tables. For example, the JJC@ test device can also simultaneously plot the fitted curve of the test data in the table. [b]2 How to test the performance of test equipment in the laboratory[/b] In laboratory testing, there is often no complex relay protection, but there are more instruments and test equipment. By conducting as many tests as possible on the test equipment, you can make a better selection for ordering. 2.1 A relatively simple method for testing amplitude-frequency characteristics is to use a two-line oscilloscope. The test setup is set to superimposed harmonics, with one phase receiving a power frequency voltage U1 and the other a high-frequency voltage Un, both with equal amplitudes (U1 = Un). Initially, U1 is simultaneously supplied to both channels of the two-line oscilloscope, and the proportional knob is adjusted to make the two waveforms the same height. Then, the high-frequency voltage Un is switched to one channel. If the amplitude-frequency characteristics are good, the two waveforms at different frequencies should have the same height on the oscilloscope. For example, with the FREJA RTS21 and OMTCRON CMC156, the amplitudes of the two waveforms are still the same at a high frequency of 1000 Hz, and the curves remain smooth at high frequencies. Due to the absence of RC smoothing, the amplitude-frequency characteristics are excellent. With the JJC@ test setup, the two waveforms are still the same height at a high frequency of 350 Hz, and at 550 Hz... At Hz, the amplitude of high-frequency voltage is about 10% lower than that of power frequency voltage. For the Beijing Weite MRT-02B test device, due to the limited number of points per cycle (40 points) and the use of significant RC smoothing measures, the amplitude-frequency characteristics are very poor. Even at the 5th harmonic, the amplitude error relative to the computer setting has been reduced by more than 50%, but further improvement is needed. 2.2 Linearity and Accuracy Measurement: Relay protection tests primarily ensure linearity and accuracy at 50 Hz. This can be verified using general ammeters and voltmeters. For current accuracy verification, the test device should be tested point by point from high current downwards to avoid overheating of devices with insufficient heat capacity. Due to the limited number of bits in the D/A converter, attention must be paid to accuracy at low voltages. For current, only the range of 0.2–30 A needs to be checked. The OMICRON CMC156, due to its A/D feedback automatic correction accuracy, has excellent accuracy and linearity. Its drawback is a relatively small output capacity. When the test setup is set to output a sine wave signal, the waveform distortion, i.e., the total harmonics, is generally required to be below 0.5%. However, at low voltages and low currents, the distortion of the test setup tends to be higher due to the limited number of bits in the D/A converter. 2.3 Load Capacity: Using a harmonic analyzer in conjunction with a load resistor, the load capacity of the test setup is measured without output waveform distortion. Sometimes, when manufacturers claim a high current output, it is essential to know the load capacity in VA, or the allowable voltage at the output port in V, and the allowable duration. For example, some devices may have a low voltage at the output port when outputting a large current, indicating poor load capacity. Others may indicate a high voltage at the output port, but not necessarily the current at that time. Generally, the load-carrying capacity should be assessed based on the port voltage drop at 5 or 10 A; otherwise, this is often not achievable when performing a full-set test of the entire protection system. 2.4 Harmonic Superposition Capability: The ability of the test device to superimpose harmonics is tested using a harmonic analyzer. This test can simultaneously assess accuracy and amplitude-frequency characteristics. 2.5 Output Square Wave and Step Wave Capability: For test devices without transformer output, the ability to output square waves and step waves can be observed using an oscilloscope. 2.6 Magnetizing Characteristic Test: Using a differential relay to perform the magnetizing characteristic test is a simple method to assess whether the test device can automatically perform the test and how it processes the test results. 2.7 Operating Region Test: Using an impedance relay to perform the operating region test is also a simple method to assess whether the test device can automatically perform the test and how it processes the test results, checking for the ability to superimpose harmonics and the ability to automatically fit and plot lines. 2.8 Precision current test: The precision current test is conducted using an impedance relay to assess the dynamic testing capability of the test device. If the transient response is poor, the results of two precision current measurements will differ significantly, especially under low current conditions. It is necessary to analyze whether the transient dispersion of the protection device or the transient characteristics of the test device is the cause. At the same time, the results of the precision current fitting and plotting should be assessed, especially in the case of data dispersion, whether the fitting and plotting will produce unreasonable oscillations. This is to assess the quality of the plotting software. [b]3 Review of software menu design[/b] (1) The initial value and fault value of the three-phase current and voltage can be set arbitrarily, harmonics can be superimposed, the length of the state time zone of each segment can be set, the information of the relay protection feedback contact can be intelligently judged, and the relay critical action value can be automatically found according to a certain mode. If the current remains constant, the voltage amplitude is changed according to the dichotomy method to find the critical operating value of the impedance element; or the current remains constant, and the voltage amplitude is reduced step by step. Under each step voltage amplitude, the voltage amplitude remains constant temporarily, and the voltage angle is continuously changed to see the angle range of the relay entering the operating area. It can be used to measure the impedance relay with the upward circular characteristic, and can also measure the operating area of ​​the directional relay. (2) The three-phase current and voltage source can be changed as needed. For example, when performing the differential protection magnetic characteristic test, one phase sends DC current and the other phase sends AC current, and the voltage can be changed to send DC voltage 220 V, etc. (3) The three-phase current and voltage source should be able to independently change the frequency so as to perform harmonic braking of the differential relay. (4) The three-phase current and voltage can also be automatically calculated and generated according to the input line parameters R, X and the selected fault type. (5) The harmonics of the three-phase current and voltage cannot be arbitrarily superimposed, because once the fundamental current and voltage have been set, it is equivalent to the line parameters R, X have been determined. Therefore, if the harmonic current is set arbitrarily first, the harmonic voltage must meet the constraints of the parameters R, X. (6) The three-phase current and voltage should also be able to set time zones for multiple states, such as normal state, fault state, non-full-phase state, reclosing to fault state, and clearing state, so as to verify the operation of the integrated reclosing. (7) The three-phase current and voltage should also be able to simulate complex fault transitions: such as normal state, single-phase ground fault state for 15 ms, then switching to two-point ground fault state. Because the first 15 ms is a single-phase ground fault, and the following is a two-point ground fault, the formula for calculating the fault distance is different for single-phase ground fault and two-point ground fault. The problem faced by the microcomputer protection at this time is that the data within one cycle is confused. Which formula should it use for calculation, or wait, or calculate in a mixed manner? Because the new relay protection is complex, its core is not very clear. How does it perform in operation? The microcomputer-type relay protection test device should shoulder the responsibility of this assessment. (8) The test device should be able to generate a three-phase current and voltage model during system oscillation, for testing purposes such as oscillation disconnection devices. It should also be able to generate a three-phase current and voltage model of system oscillation accompanied by a fault, including models of fault before oscillation or oscillation before fault. (9) Advanced simulation: The EMTP electromagnetic transient program is used to calculate fault simulation data, which is converted into a data file of the test device in a certain format. The three-phase simulated current and voltage are sent out by D/A amplifier with power amplification. For test devices without isolation transformers that use computer information to transmit data step by step and have more than 40 points per week, it is not difficult to achieve this. The more points per week, the more detailed the simulation. (10) In principle, the data recorded by the fault recorder should be able to be reproduced and output through the test device to realize the reproduction of fault information. The portability and standardization of the device, its appearance and workmanship, and the reliability of the connectors are all selection criteria.